Detection of fluids with metal-insulator-semiconductor sensors

ABSTRACT

A MIS diode sensor having a Pd/Cu alloy electrode is sensitive to and can detect CO, H 2 , C 2  H 2  and C 2  H 4 . An array of MIS diode sensors having varying electrode compositions can indicate a fault in an electrical transformer by the detection of at least one of the key fault gases CO, H 2 , C 2  H 2  and C 2  H 4 .

TECHNICAL FIELD

The present invention relates in general to methods and devices for thedetection of specific components of gases in liquid or gaseous fluidwith metal-insulator-semiconductor (MIS) microelectronic sensors. In oneembodiment, the invention relates to methods and devices for thedetection in oil-filled electrical transformers of specific fault gasesusing selective MIS microelectronic sensors.

BACKGROUND OF THE INVENTION

Methods for industrial gas and vapor analysis have evolved along twoseparate paths. The first involves complex instruments for exampleinfrared spectroscopy, gas chromatography, and mass spectrometry. Thedevelopment of microcontrollers and microcomputers has led to smallerand more rugged versions of these analytical instruments that were onceonly found in laboratories. These instruments are very powerful, butthey have disadvantages. They are costly and maintenance-intensive. Theyare usually located far from the gas or vapor source in climatecontrolled enclosures. The gas or vapor sample is usually transported tothe analyzer in a heated tube. This process delay will not provide realtime information. These instruments are not likely to be affordablecandidates for real-time, in-situ applications such as combustion gasanalysis or chemical process analysis. The second evolutionary path isthe development of emerging chemical sensor technology.

There are many industrial applications for gas and vapor sensortechnology. For example, there is the detection of hazardous gas in theworkplace for worker safety, the analysis of products in the combustionof fuel for better control of the air-fuel mixture and the analysis offeed and product streams for the optimization of product yield and wastereduction.

The use of sensors in workplace gas detection instruments is routine.The technologies used are individual sensors which are installed todetect a single gas or vapor whose concentration, in the localenvironment, may approach a hazardous level. The examples of these gasesinclude hydrogen sulfide, carbon monoxide, chlorine, ammonia, hydrogen,methane and many others.

The development of sensors is ongoing for use in aerospace applicationswhere weight and size are critical issues. For example, the magnitudeand location of hydrogen leaks in liquid fueled rockets is important inlaunch readiness applications.

The use of gas sensors in combustion analysis to control fuel airmixtures began with the use of a solid state electrochemical sensor foroxygen. The sensor was designed to detect the precise air-fuel mixturein automobiles that would not be too rich or too lean. Additionalsensors to analyze the mixture for carbon monoxide and oxides ofnitrogen are important in fine tuning the mixture.

The use of chemical sensors in chemical process streams has thepotential of improving process efficiency and the resulting yield. Adirect result of process improvement is the potential to reduce theamount of waste. For example, the water concentration in a siliconeprocess feed stream is an important parameter in the final productquality.

The degree to which these sensors can uniquely identify the gas or vaporthey were designed to detect depends on the sensitivity of the sensor toother interfering species and the concentration of those species. Theability to resolve the target gas or vapor is called the selectivity.There are very few examples of sensors which are highly selective (e.g.greater than a ten fold difference in relative sensitivity), and eventhese do not work ideally. For example, a sensor whose target gas willgive an incorrect indication of the target gas when the concentration ofthe interferant is high enough. In reality, this limitation is relievedby using the sensor in a situation where the situation described aboveis unlikely.

An additional way to improve the quality of the information provided bya sensor is to process information from an array of sensors.

One application for chemical sensors is in the detection of particulargases in mineral oil-filled electrical transformers.

Faults such as arcing, corona discharge, low energy sparking, severelyoverloading, pump motor failure and overheating in the insulation systemcan generate hydrogen (H₂), acetylene (C₂ H₂), ethylene (C₂ H₄), andcarbon monoxide (CO) in oil-filled electrical transformers. Theseconditions can result in transformer malfunction and may eventually leadto failure if not corrected. The detection of the presence of these fourtransformer fault gases is frequently the first available indication ofa possible malfunction. There is a statistical correlation betweentransformer malfunction conditions and the fault gases they generate.This correlation has enable the evaluation of possible fault types withwhich by a key gas method.

The significant gases (key gases) and the four general fault types withwhich they are associated are as follows:

a.) Thermal-Oil--C₂ H₄,

b.) Thermal-Cellulose--CO₂, CO

c.) Electrical Corona--H₂

d.) Electrical-Arcing--H₂, C₂ H₂

Because of this correlation, the possible type of fault in a transformercan be evaluated by the analysis of the gases generated in thattransformer.

The utility industry has developed various diagnostic theories thatemploy ratios of certain key gases. The existence of such "gas ratios"makes it possible to improve the reliability of transformers. However,in order to evaluate the possible fault types using the various gasratios (e.g. Rogers Ratio Method, Doernenburg Ratio), the gases must bedistinguished and analyzed.

Currently gas chromatography is used to identify and quantify gasesdissolved in oil having a viscosity of 20 centistokes or less at 40° C.(104° F.). A significant disadvantage, to the user, is the lack ofreal-time analytical information.

For gas chromatography to be effective, the quantity and composition ofgases dissolved in oil samples must remain unchanged during transport tothe laboratory. Sampling necessarily involves the risk of exposing theoil to air and the concomitant loss of dissolved gases.

An alternative would be a chemical sensor immersed directly into the oilenvironment of the electrical transformer, operating on a real-timebasis to indicate the presence of an incipient fault, and which wouldnot require sampling the oil and subsequent analysis in a laboratory.

Selective sensors capable of distinguishing and analyzing particularcomponents would exhibit a sensitivity (output/concentration unit) to aparticular component that is different in magnitude than the sensitivityto a different component giving a signal.

The sensor response in a mixture of two components should ideally be asimple sum of the response (i.e. voltage) in the individual components.In this linear case, the effects can be resolved conventionally with ananalytical algorithm such as one based on matrix algebra as taught by L.W. Potts for chemical analysis in Quantitative Analysis: Theory andPractice, Harper and Row, New York 1987, Chapter 13.

Chemically sensitive field-effect transistors (CHEMFETs) have beendeveloped for the detection of specific compounds in liquid and gaseousenvironments, such as the ion sensitive CHEMFETs disclosed in U.S. Pat.No. 4,020,830 to Johnson, et al. and U.S. Pat. No. 4,305,802 toKoshiishi.

Other CHEMFETs have been produced that measure the concentrations ofcomponents in a gaseous state, as for example the devices disclosed inU.S. Pat. No. 3,719,564 to Lilly, Jr., et al. and described by Shimada,et al. in U.S. Pat. Nos. 4,218,298 and 4,354,308, and the suspended gatefield-effect transistors (SGFETs) described by Jiri Janata in U.S. Pat.Nos. 4,411,741 and 4,514,263. These devices are relatively complex andcostly to manufacture due to their multiple junctions and diffusionregions.

CHEMFETs in general, and SGFETs in particular are also not well suitedto detection of combination of specific compounds or specific compoundsin the presence of other potentially interfering chemical species.Combinations of discrete SGFETs with sensitivities to differentcompounds have been proposed in efforts to address such deficiencies.For example, in U.S. Pat. No. 4,368,480 to Senturia multiplexed CHEMFETsprovide logic elements with varying on-off duty cycles. Nevertheless,such combinations are even more difficult and costly to manufacture thanindividual sensors.

A device and method for detection of fluid concentration utilizingcharge storage in a MIS diode is disclosed in U.S. Pat. No. 4,947,104 toPyke. MIS sensors detect and amplify the change in the work function ofits metal electrode when gas adsorbs on its surface. A MIS sensor forhydrogen was described with a palladium metal electrode by M. S.Shiveraman et al., Electron Lett., 12,483 (1976) and Z. Li et al.,IEEE/IEDM-85,125 (1985). MIS sensors with Ni and Pt electrodes formethane and carbon monoxide have been reported by T. L. Poteat, et al.J. Electron. Matl., 12,181 (1983).

The MIS tunnel junction was developed with a palladium electrode forhydrogen detection as reported by R. C. Hughes et al., J. Appl. Phys.62(3), Aug. 1, 1987 (pp 1074-1083). Operated in reverse bias with anoxide layer thin enough for a measurable reverse current, this sensor ismuch more sensitive to hydrogen. In this example, the work functionchange is due to a layer of Pd-H dipoles at the metal insulatorinterface and due to a change in the composition of the metal onadsorbing hydrogen.

It is therefore desirable to provide methods and devices for detectingand analyzing specific gases in a mixture. It is further desirable toprovide methods and devices to indicate the presence of the key faultgases in mineral oil, predict their generation rate, and alert thesubstation operator accordingly.

SUMMARY OF THE INVENTION

The present invention provides an MIS diode sensor having an electrodecomprising an alloy of palladium and copper for detecting components ina fluid.

The present invention further provides an MIS diode sensor for detectingacetylene, said sensor having an electrode comprising a compositionselected from the group consisting of platinum, platinum/tin alloy,palladium/silver alloy, palladium/copper alloy, palladium/chromiumalloy, nickel/chromium alloy, chromium and mixtures thereof.

The present invention further provides a device for detecting andanalyzing at least two selected fluids in a mixture comprising an arrayof at least two MIS diode sensors including a first MIS diode sensorhaving a first electrode with a higher sensitivity in a mixture to atleast a first fluid and a lower sensitivity in the mixture to at least asecond fluid, and a second MIS diode sensor having a second electrodewith a higher sensitivity in a mixture to at least the second fluid anda lower sensitivity in the mixture to at least the first fluid. Thefirst electrode has a work function proportional to the concentration ofat least the first fluid, producing a corresponding flat-band voltage.The second electrode has a work function proportional to theconcentration of at least the second fluid, producing a correspondingflat-band voltage. The device further comprises a circuit fordetermining the magnitude of the flatband voltage of each sensor, and aprocessor for calculating at least the first fluid concentration.

The present invention further provides a device for indicating a faultin an electrical transformer comprising an array of at least a first, asecond and a third MIS diode sensors, wherein said first sensor has anelectrode comprising platinum, said second sensor has an electrodecomprising palladium/copper alloy and said third sensor has an electrodecomprising palladium/silver alloy. The device further comprises acircuit for determining the magnitude of the flat-band voltage of eachsensor in response to the presence of a concentration of a fault gas towhich at least one of the sensors is sensitive, and a processor forcalculating the fault gas concentration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view through an exemplary metal insulatorsemiconductor diode embodying the concepts of the present invention.

FIG. 1A is an elevational view through an alternative embodiment of theinventive metal insulator semiconductor diode.

FIG. 2 is a plan view of a portion of an array of metal insulatorsemiconductor diodes embodying the concepts of the present invention andcorresponding circuitry.

FIG. 3 is a graphical representation of the response to hydrogen of aplatinum electrode MIS sensor in mineral oil.

DETAILED DESCRIPTION OF THE INVENTION

A device in accordance with the present invention for monitoring theconcentration of at least one selected fluid, such as an electroactivegas, includes a metal insulator semiconductor diode sensor having a workfunction proportional to the selected fluid concentration and a changein flat-band voltage (V_(fb)), proportional to the work function.Circuitry is provided for determining the magnitude of the change inV_(fb) from which the selected fluid concentration may be calculated.

FIG. 1 illustrates a device generally indicated by the numeral 10, whichalso embodies a method for monitoring the concentration of at least oneselected liquid or gaseous fluid. Device 10 is and shall hereinafter bereferred to as a metal insulator semiconductor (MIS) diode.

MIS diode 10 may be formed of a semiconductor substrate 11 having p or ntype doping characteristics, a layer of dielectric, or insulator 12 suchas silicon dioxide and/or silicon nitride covering substrate 11 andsacrificial layers 13 and 14 applied to the surface of insulator 12opposite substrate 11. Sacrificial layers 13 and 14 may be maderespectively of a fugitive metallic material such as a thin film ofaluminum and tungsten/titanium, or other conventional sacrificialmaterials such as tungsten, glass or organic materials. The fugitivesacrificial layer 13 and thin film sacrifical layer 14 are etched orotherwise formed to provide a cavity 16 into which the selected fluidmay flow after passage through a plurality of holes 17 in suspendedcatalytic electrode 15. In use, the selected fluid, such aselectroactive gas, adsorbs onto the electrode, inducing the responsivechange in flat-band voltage.

Substrate 11 may alternatively be comprised of a semiconductor such asgallium arsenide, indium phosphide or titanium dioxide, and insulator 12may be comprised of alumina, silicon nitride or aluminum nitride. Afurther alternative embodiment and its fabrication will be describedwith regard to FIG. 1A, below.

Where greater noise immunity, redundance, or ability to detectcombinations of specific components or specific components in thepresence of other potentially interfering chemical species is desired,it is preferable to integrally form a plurality of MIS diodes in anarray together with suitable circuitry to selectively determine thecontribution to Vfb for each component of the mixture.

A plurality of metal insulator semiconductor diodes, each of whose workfunction is variously proportional to each of the selected fluidcomponent concentrations and whose V_(fb) is proportional to theconcentrations, may be connected to a microprocessor for later computingeach of the component concentrations.

As depicted in FIG. 2, each MIS diode 10 which forms a part of array 30is electronically connected to an analog circuit 31 foranalog-to-digital conversion, by wire conductors 34. The array 30 may bemounted on an alumina substrate 40 to control temperature and providefor electrical isolation. Fluid concentrations are calculated from thedigital signal relayed through electrical pathway 32 using amatrix-based algorithm installed in a microprocessor 33, such as aMotorola HC11 microprocessor. The real-time gas concentration iscommunicated by an indicator, such as being displayed on a liquidcrystal display 35 and/or being made available through an RS232 or fiberoptic connection 36 for remote access such as by modem 37. Each MISdiode 10 may be provided with suspended electrode materials suitable forsensitivity to a desired fluid. In this manner a multiple gas and/orimproved reliability detector may be achieved.

It should be noted that the MIS diode sensors described herein canoperate either in the gas phase or immersed in a liquid such as mineraloil, and do not require an electrolyte or oxygen for operation.

In one embodiment of the invention, the composition of the suspendedelectrode 15 is provided which is selective in its response to acomponent fluid, or fault gas, indicative of the condition of theelectric transformer. In a preferred embodiment of the invention, acombination of suspended electrode compositions is utilized in an arrayof associated MIS diodes, resulting in a more reliable measurement ofeach of the gases. This is due to the electrode compositions exhibitingvarying sensitivity to more than one component fluid, or fault gas, inthe fluid mixture, or oil. If each of the electrode compositions weresensitive to only a particular component fluid, or gas, the reliabilityof the measurement would be solely dependent upon each individualsensor. When MIS diode sensors having different electrode compositionsare utilized in an associated array, each sensor can be used as a checkon the other(s).

In a preferred embodiment of the invention, MIS diode sensor electrodecompositions selected for individual transformer faults and theircorresponding symptomatic gases, and more preferably for use in anassociated array are listed in Table I. These sensor electrodecompositions are not affected by, and thus do not respond to thetransformer mineral oil, saturated hydrocarbons, carbon dioxide ornitrogen.

                  TABLE I                                                         ______________________________________                                                                     PRINCIPAL                                                                     ELECTRODE                                        FAULT                        COMPO-                                           TYPE    PRINCIPAL GAS        SITION                                           ______________________________________                                        Overheated                                                                            C.sub.2 H.sub.4, H.sub.2                                                                           Platinum                                         Oil     (Also C.sub.2 H.sub.6, with smaller quantities                                of H.sub.2 and CH.sub.4 may form. Traces of                                   C.sub.2 H.sub.2 may form if fault is severe                                   or involves electrical contacts.)                                     Overheated                                                                            CO                   Palladium/                                       Cellulose                                                                             (Also, C.sub.2 H.sub.6 and C.sub.2 H.sub.4 may form                                                Copper                                                   fault involves oil-impregnated                                                structure.)                                                           Corona  H.sub.2              Palladium/Silver                                         (Also CH.sub.4, C.sub.2 H.sub.6 and C.sub.2 H.sub.4.                          Comparable amounts of CO and                                                  CO.sub.2 may result from discharges                                           in cellulose.)                                                        Arcing  H.sub.2, C.sub.2 H.sub.2                                                                           Platinum                                                 (Also, CH.sub.4, C.sub.2 H.sub.6 and C.sub.2 H.sub.4. CO                      and CO.sub.2 may form if the fault                                            involves cellulose. Oil may be                                                carbonized.)                                                          ______________________________________                                    

The fabrication and testing of microelectronic MIS diode sensors havingelectrodes including the above compositions for target key fault gasesare described below.

Fabrication

Alternative embodiment MIS diodes, having the structure shown in FIG. 1Awere fabricated as follows. The sensor wafer fabrication process beganwith a p-type silicon substrate 11. The substrate 11 was cleaned in HF,and a 300 Å silicon oxide insulator layer 12 was grown in O₂ /N₂ at hightemperature. Separate field oxide and gate oxide zones may be formed ifdesired, to reduce noise by isolating the column of silicon under theelectrode.

A 300 Å isolation layer 18 of Si₃ N₄ was deposited on the oxide by a lowpressure chemical vapor deposition (LPCVD) process. A lift-offphotoresist was deposited, exposed and developed. A sacrificial layer 13of titanium tungsten alloy (400 Å) was sputtered onto the siliconnitride isolation layer 18 to provide a satisfactory bonding surface foradhesion of the deposited noble metal electrode 15. The photo-resist wasdissolved to lift off the unwanted electrode material and resolve theelectrode 15 and hole 17 patterns.

The wafers were etched in a solution of 0.1 moles per liter EDTA held ata pH=9.5 to 10.5 at room temperature for 5-10 minutes. The etch removesthe TiW to form cavity 16. With this process the adhesion to the nitridelayer was achieved by metal columns that were not completely etched.

The etched wafers were sawed and cleaved. Individual sensor die weresecured and contacted electrically to headers with silver epoxy.

In an alternative embodiment, aluminum can be utilized rather than TiW.In that instance, an etch process in PNA (1,600 ml H₂ O, 100 mlphosphoric acid, 100 ml nitric acid, and 100 ml acetic acid, diluted25:1 with water) removes the aluminum from underneath the electrode.

Example 1

An automated vapor dilution and delivery system (VG7000, MicrosensorSystem, Inc.) was used to introduce test gases to the electrodes. Thesystem was programmed to switch between sources of analyzed target gasdiluted in nitrogen (AIRCO Special Gases; Division of BOC, Inc.) andpure nitrogen. The sensors were tested at between 50° and 100°Centigrade in an environmental chamber (Ransco 900 series, Despatch,Inc.), where the sensors (on TO-8 headers) were pressed into a Vespel™(DuPont) test fixture with low dead volume. Temperature conditioning ofthe feed gas was achieved by flowing gas through a one-meter length of1/4-inch diameter copper coil inside the environmental chamber prior toexposure to the sensor.

A microelectronic MIS diode sensor having an electrode composition ofplatinum (Pt) was coated with mineral oil and sequentially exposed tohydrogen (H₂), acetylene (C₂ H₂), ethylene (C₂ H₄), and carbon monoxide(CO) in a nitrogen (N₂) carrier gas. This sensor test demonstrated thatthe Pt electrode was responsive to H₂, C₂ H₂ and C₂ H₄, and moreimportantly, that the signal emitted by the Pt electrode sensor clearlydistinguished the three gases from carbon monoxide (CO).

The sensitivity (volts/ppm) of each of the suspended catalyticelectrodes was determined by exposing each of the sensors to increasingconcentrations of each of the key gases. In a typical experiment, theconcentration was held at 1 ppm in a background of nitrogen for 15minutes to one hour. The temperature was held constant. The sensor wasallowed to reach equilibrium at constant concentration during which timethe voltage output was recorded. The concentration was increased, andthe experiment was repeated. The sensitivity proved to be linear in thelog of the concentration (ppm) over the ranges of interest for theanalysis of the key gases.

It is preferred that the MIS sensor electrodes which are to be used todetect acetylene or ethylene be exposed to hydrogen or a similarreductant after exposure to air or oxygen, in order to remove adsorbedoxygen from the electrode surface. An exposure to hydrogen at a doserate of about 1 ppm.hour was found sufficient to react with adsorbedoxygen to form water, which was then desorbed from the electrode metalsurface.

In the example of platinum responding to the key gases at 60 degreescentigrade, the sensitivity to hydrogen was 2.5 times greater than toeither acetylene or ethylene and at least 10 times greater than tocarbon monoxide in the concentration ranges of interest. An example ofthe response to hydrogen at 3 different concentrations of a platinumelectrode MIS sensor is represented by the graph shown in FIG. 3.

A comparison of the selectivity and sensitivity of Pt electrode sensorsto the various target gases is shown in Table II below. It is theorizedthat the effect on the magnitude and sign of the output are proportionalto the strength and polarity of the to component bond.

                  TABLE II                                                        ______________________________________                                        Target Gas      Platinum MIS                                                  ______________________________________                                        Hydrogen        100                                                           Carbon Monoxide <10                                                           Acetylene       40                                                            Ethylene        40                                                            ______________________________________                                    

Using four or more sensors with electrodes of varying composition, thesensitivities to the various key gases were found to vary such that incombination, in an analysis algorithm as referred to above, thecomposition of each of the key gases can be computed. In one example, asystem of linear equations can be used to compute the concentrations. Ina preferred embodiment, a matrix formulation of the sensitivities iscombined with experimentally determined constants to compute theconcentrations. Any other formulation of algorithm to compute theconcentrations from the inputs of variously sensitive MIS sensors orCHEMFETS with catalytic suspended electrodes could also be used.

Example 2

A microelectronic MIS diode sensor having an electrode composition of analloy (about 50 atomic percent) of palladium and copper (Pd/Cu) wastested for its response in oil to carbon monoxide (CO) at a level of2000 ppm in nitrogen. The Pd/Cu electrode sensor responded to that COconcentration with an output of 800 millivolts (mV) at 100° C.

In the Pd/Cu electrode sensor, we have found that hydrogen sensitivityis less than 10% of the sensitivity to CO. The Pd/Cu electrode sensorthus exhibited the selectivity necessary to identify and quantify CO inmixtures containing H₂.The Pd/Cu sensor responded with greatersensitivity to CO than it did to the other key fault gases.

The MIS diode sensor having a palladium/copper electrode demonstrated aresponse, although with different sensitivities, to hydrogen, carbonmonoxide, and unsaturated gaseous hydrocarbons such as ethylene andacetylene. Such a Pd/Cu electrode sensor could be used as a detector forany of these fluids in a system where the other listed response-inducingfluids are absent, or where the other fluids' presence and concentrationare detectable and calculable using a different sensor (as a reference).The preferred atomic ratios of the components of the Pd/Cu alloy rangefrom about 1Pd:3Cu to about 3Pd:1Cu.

Example 3

A microelectronic MIS diode sensor having an electrode composition of analloy about 25 atomic percent of palladium and silver (Pd/Ag) was testedfor its response to target fault gases. In the test of palladium/silverresponse to the key gases at 60 degrees centigrade, the sensitivity tohydrogen was about 1.2 times greater than to acetylene and about 1.4times greater than to ethylene, and at least 10 times greater than tocarbon monoxide in the concentration ranges of interest.

Alternative Embodiments

For electrical transformer fault gases, the key gas concentrations inthe following ranges were tested with various MIS electrode compositionsto determine response.

    ______________________________________                                        Hydrogen             3-3,000 ppm                                              Acetylene            1-150 ppm                                                Ethylene             3-300 ppm                                                Carbon Monoxide      20-2,000 ppm                                             ______________________________________                                    

In a preferred embodiment, an array of three MIS diode sensors wasfabricated each having a different electrode composition from Table I.In addition to the different associated sensors distinguishing bothdifferent and some similar gases and thus being able to serve asreferences for one another, the range of selectivity is improved withdifferent sensors having different sensitivity to the same gas, andreliability is improved.

Alternative MIS diode electrode compositions which are responsive to andselective for key fault gases are listed below in Table III.

                  TABLE III                                                       ______________________________________                                                           ELECTRODE                                                  PRINCIPAL GAS      COMPOSITION                                                ______________________________________                                        C.sub.2 H.sub.4    Platinum/Tin                                                                  Palladium/Silver                                                              Palladium/Copper                                           CO                 Nickel/Chromium                                                               Palladium/Silver                                           H.sub.2            Platinum                                                                      Palladium/Nickel                                                              Palladium/Chromium                                                            Palladium/Copper                                           H.sub.2, C.sub.2 H.sub.2                                                                         Chromium                                                                      Platinum/Tin                                                                  Palladium/Copper                                                              Palladium/Silver                                                              Palladium/Chromium                                                            Nickel/Chromium                                            ______________________________________                                    

The MIS diode sensors described above can be mounted onto a probe andplaced in the sample of the fluid to be tested but preferably aremounted in-situ in order to provide the capability for continuouslymonitoring the condition of the fluid, such as electrical transformeroil within the sealed containment vessel. The electronically connectedtransducer and microprocessor can be mounted outside the containmentvessel for protection from the oil and for easy access.

The MIS diode sensors described above can also be used to detect andanalyze the concentration of hydrogen, carbon monoxide, alkenes such asethylene, and alkynes such as acetylene in mixtures of gases or fluidssuch as chemical process feed, product or waste streams.

Inasmuch as the present invention is subject to variations,modifications and changes in detail, a number of which have beenexpressly stated herein, it is intended that all matter describedthroughout this entire specification or shown in the accompanyingdrawings be interpreted as illustrative and not in a limiting sense. Itshould thus be evident that a device constructed according to theconcept of the present invention, and reasonably equivalent thereto,will accomplish the objects of the present invention and otherwisesubstantially improve the art of monitoring the concentration ofselected liquid or gaseous fluids.

What I claim is:
 1. A metal-insulator-semiconductor diode sensor havingan electrode comprising an alloy of palladium and copper for detectingcomponents in a fluid.
 2. The metal-insulator-semiconductor diode sensoras in claim 1 wherein the atomic ratio of palladium to copper is betweenabout 1Pd:3Cu and 3Pd:1Cu.
 3. The metal-insulator-semiconductor diodesensor as in claim 1 wherein the atomic ratio of palladium to copper is1:1.
 4. A metal-insulator-semiconductor diode sensor for detectingacetylene, said sensor having an electrode comprising a compositionselected from the group consisting of platinum/tin alloy,palladium/chromium alloy, nickel/chromium alloy, chromium and mixturesthereof.
 5. A device for detecting and analyzing at least two selectedfluids in a mixture comprising an array of at least twometal-insulator-semiconductor diode sensors including a firstmetal-insulator-semiconductor diode sensor having a first electrode witha higher sensitivity in a mixture to at least a first fluid and a lowersensitivity in the mixture to at least a second fluid, and a secondmetal-insulator-semiconductor diode sensor having a second electrodewith a higher sensitivity in a mixture to at least the second fluid anda lower sensitivity in the mixture to at least the first fluid.
 6. Thedevice as in claim 5 wherein the first electrode has a work functionproportional to the concentration of at least the first fluid, producinga corresponding flat-band voltage and the second electrode has a workfunction proportional to the concentration of at least the second fluid,producing a corresponding flat-band voltage.
 7. The device as in claim 6further comprising a circuit for determining the magnitude of theflat-band voltage of each sensor.
 8. The device as in claim 7 furthercomprising a processor for calculating at least the first fluidconcentration.
 9. A device for indicating a fault in an electricaltransformer comprising an array of at least a first, a second and athird metal-insulator-semiconductor diode sensors, wherein said firstsensor has an electrode comprising platinum, said second sensor has anelectrode comprising palladium/copper alloy and said third sensor has anelectrode comprising palladium/silver alloy.
 10. The device as in claim9 further comprising a circuit for determining the magnitude of theflat-band voltage of each sensor in response to the presence of aconcentration of a fault gas to which at least one of the sensors issensitive.
 11. The device as in claim 10 further comprising a processorfor calculating the fault gas concentration.
 12. A device for indicatinga fault in an electrical transformer containing a fluid wherein thefault is indicated by a concentration of a gas in the transformer fluid,comprising an array of i) a first metal-insulator-semiconductor diodesensor having an electrode principally sensitive to carbon monoxide, ii)a second metal-insulator-semiconductor diode sensor having an electrodeprincipally sensitive to hydrogen, and iii) a thirdmetal-insulator-semiconductor diode sensor having an electrodeprincipally sensitive to ethylene and acetylene, wherein each of therespective electrodes of each of said first sensor, second sensor andthird sensor comprise a different composition than the other electrodes.13. A device as in claim 12, wherein the electrode of the first sensoris selected from the group consisting of a Pd/Cu alloy, a Pd/Ag alloyand a Ni/Cr alloy.
 14. A device as in claim 12, wherein the electrode ofthe second sensor is selected from the group consisting of Pt, a Pd/Agalloy, a Pd/Ni alloy, a Pd/Cr alloy and a Pd/Cu alloy.
 15. A device asin claim 12, wherein the electrode of the third sensor is selected fromthe group consisting of Pt, a Pt/Sn alloy, a Pd/Ag alloy, a Pd/Cu alloy,Cr, a Pd/Cr alloy and a Ni/Cr alloy.
 16. A device as in claim 12,further comprising a circuit for determining the magnitude of flat-bandvoltage of each sensor in response to the concentration of the gas. 17.A device as in claim 16, further comprising a processor for calculatingthe concentration of the gas.
 18. A device as in claim 17, furthercomprising an indicator for communicating the concentration of the gas.19. A method for detecting at least one component in a fluid utilizing ametal-insulator-semiconductor diode sensor, wherein said at least onecomponent is selected from the group consisting of carbon monoxide,hydrogen, acetylene, ethylene and mixtures thereof, comprisingprovidingsaid sensor having an electrode comprising an alloy of palladium andcopper, contacting the electrode with the fluid, and measuring thesensitivity of the electrode to said at least one component in thefluid.
 20. A method for detecting acetylene in a fluid utilizing ametal-insulator-semiconductor diode sensor comprisingproviding saidsensor having an electrode comprising a composition selected from thegroup consisting of platinum, platinum/tin alloy, palladium/silveralloy, palladium/copper alloy, palladium/chromium alloy, nickel/chromiumalloy, chromium and mixtures thereof, contacting the electrode with thefluid, and measuring the sensitivity of the electrode to acetylene.